1. Field of the Invention
The present invention relates to an automatic focus control device for use in a camera system, more particularly, to an automatic focus control device having an auxiliary light source for radiating auxiliary light to an object in case of low brightness and/or low contrast of the object.
2. Description of Prior Art
There is provided a camera system comprising an automatic focus control device which detects a focus condition such as the amount of defocus and the direction of defocus in accordance with a light passing through a photograph lens of the camera from an object to be focus controlled, and controls an automatic focus adjustment in accordance with a result of the focus condition detection. In this sort of automatic focus control device, it is possible to detect an in-focus position correctly, when the amount of defocus or a defocus value, corresponding to a difference quantity between a current position and an infocus position of a photograph lens of the camera, is only within an available range of the focus detection. On the other hand, it is impossible to detect the in-focus position when the defocus value is out of the detection available range, in this case, a low contrast search control is performed as follows. That is, the focus condition is detected moving the photograph lens into another position, and the in-focus position of the object is detected when the photograph lens is positioned in the detection available range.
Conventionally, there are known focus detecting device in which an auxiliary light is radiated to an object to be focus detected from an auxiliary light source so as to detect the correct focus condition under the low brightness of the object.
Besides, a photographic lens has a color aberration which is a displacement of the focus position due to wave length of light. In case the focus detection is made using an infra red or a red auxiliary light there occurs a difference of the focus position due to the color aberration between a focus position with the light wave length of the auxiliary light for the focus detection and a focus position with a reference wave length of the ambient light for the photographing.
To compensate the difference of the focus detection, Japanese laid open patent No. 43620/1985 proposes a focus detecting device which enables to obtain a correct focus position by using a property that the aberration on a lens axis changes linearly within a wave length range including infra red light. In the proposed arrangement, one or more data of the color aberrations on the lens axis in a specific wave length is stored in a register of the focus detecting device, then the color aberration of the wave length of the light used for the focus detection is obtained based on the stored color abberation data, the specific wave length, the wave length of the light at the time of focus detection and the reference wave length. Thus a focus position obtained by the measurement using the auxiliary light is subtracted by the obtained color aberration so that a correct focus position can be obtained.
However, in the above method, there must be provided optical filters on the respective optical systems of either the auxiliary light emitting side and light receiving side. Furthermore, there must be provided an arrangement of switching light filters in the light receiving optical system corresponding in accordance with the focus detection under ambient light and under auxiliary light. Hence, the device becomes complicated and expensive.
There is disclosed in Japanese Laid Open Patent publication No. 59413/1983 another focus detection device in which the difference of the focus positions between the ambient light and the infra red light by a calculation .DELTA.d .times. (color aberration of the infra red light)/(color aberration of the ambient light) so that a correct automatic focus can be obtained under any light having any wave length. .DELTA.d represents the difference of the focus position between the ambient light of a specific wave length and infra red light.
In the arrangement, there must be provided another set of light receiving element and its connecting terminals for deriving the signal from the light receiving element in addition to the light receiving element for detecting the contrast of the object, whereby the arrangement is not suitable to decrease the size of the arrangement and production cost. Moreover, there occurs an excessive correction for an output of the photosensor array of the automatic focus control in case of the object with a high contrast and low output for the ambient light and a low contrast and high output for the infra red light.